Dewar Benzene and Some of its Derivatives. A photoelectron spectroscopic analysis [1]

Abstract

The near degeneracy of the two lowest ionization energies of Dewar benzene can be understood in terms of closely competitive ‘through‐bond’ and ‘through‐space’ interaction. Empirical, semiempirical, and open‐shell ab initio procedures converge to require mutually consistent symmetry assignments.

Direct Characterization of Hexamethyl(Dewar Benzene) Radical Cation by Electronic Absorption Spectroscopy

“…An assignment of the 930 nm band as due to matrix radical cations formed in the primary process of radiolysis can be ruled out since this band is observed in two different matrices whose radical cations show very different spectra (radical cations generated from 2-chlorobutane are very well characterized and absorb at ≈540 nm). , Also, ionization of 2 under the same experimental conditions indicated complete hole transfer from the matrix to the solute. For the same concentration of 1 , and taking into consideration that the ionization potential of 1 is lower than that of 2 , , the processes of charge transfer should be completed with similar efficiency.…”

“…In contrast, absorption of light will promote an electron from the bonding (a 1 , HOMO-1) to the antibonding combination of π-bond orbitals (b 2 , HOMO), which results in a weakening of the attractive interactions and hence to a flattening of the structure. Additionally, the radical cation that results from removal of an electron from this orbital (a 1 ) possesses considerably weakened transannular bond. , As a result the energy gap between these orbitals should widen on going to the radical cation.…”

“…The radical cations of the valence isomers, hexamethyl(Dewar benzene) 1 and hexamethylbenzene 2 , have long been the target of interest. − Particular efforts were undertaken to provide evidence for the existence of the primary radical cation of 1 . A first unambiguous though indirect proof was delivered by Roth et al by means of the chemically induced dynamic nuclear polarization (CIDNP) technique.…”

The Dewar Benzene Radical Cation and Its Ring-Opening Reaction

“…Nevertheless, the transformation is orbital symmetry forbidden along this pathway, similar to the neutral 1 → 2 rearrangement where the ground state of 1 correlates with a doubly excited state of 2 and vice versa . Thus, the question arises whether 1 •+ has a way to avoid the high barrier that it faces along this orbital symmetry-forbidden pathway for its decay to 2 •+ …”

“…A subtle but important point to be considered in the case of Dewar benzenes is, however, that their radical cations have two very close-lying ground states which are attained, respectively, by ejection of an electron from the antibonding combination of olefinic π-MOs ( 2 B 2 ) or from a totally symmetric MO that represents a mixture of the bonding combination of olefinic π-MOs and a σ CC -MO that is located in the central bridge ( 2 A 1 ) . The photoelectron spectrum of 1 shows that these two states are nearly degenerate at the geometry of neutral 1 , but Bieri et al concluded from their analysis of this spectrum and from different calculations that the ground state of 1 •+ (at the geometry of neutral 1 ) has B 2 symmetry . If this is the case, then the ground state of 1 •+ and 2 •+ have the same symmetry (actually, 2 •+ has a degenerate ground state in D 6 h , the components of which can be classified as 2 B 1 and 2 B 2 within the C 2 v point group and which interconvert very rapidly on the very flat Jahn−Teller surface of 2 •+22 ).…”